Such planetary transmissions, mostly built from single- or multiple- stage planetary transmissions, are basically implemented in machine tools, as well as in lathes, milling machines and machining centers. The selectable planetary transmission increases the service scope of the main spindle motors, as well as the flexibility of the machine tool for processing the different work materials through increasing high torques or high speeds. For example, two transmission stages may be built in, while on one hand, the drive shaft which is connected to a sun gear with an internal gear and on the other, the internal gear can be coupled to the housing. A planetary carrier is connected to the output shaft.
In DE A 199 17 673 of the applicant, there is such a two-stage planetary transmission, with a drive shaft and an output shaft which are mounted in the housing over roller bearings. The drive shaft is connected to a sun gear which engages with the planetary gears, which are mounted over bearings on planetary bolts in a planetary carrier, which is connected over a positive-fit connection with the output shaft. The planetary gears are combed with an internal gear, which is mounted over the roller bearings in the housing or in the rotating planetary carrier and positively fit with a hub connection. The hub is torque-proof connected over an external gearing with the first part of the sliding collar.
A roller bearing is located over the sliding collar. The external part is axially displaceable in the housing, running over bearing bolts, whereby the axial displacement of the external part is carried over the roller bearings.
The sliding collar displacement takes place by means of a switching device, which is actuated by a solenoid or by a motorized switching unit whose armature is connected with the sliding collar.
This kind of planetary transmission has multiple rotating elements and, consequently, many bearings which are lubricated with the appropriate oil. Most of the traditional planetary transmissions for machine tools use splash lubrication, which has the disadvantage that it may be tainted with high mix losses and that the maximum possible rotational speed of the internal gear is limited by the internal gear bearings as they get warm, as the absorbed heating cannot be released. It is possible to cool the splash lubrication oil with circulatory lubrication, without avoiding the mix losses.
Further planetary transmissions are built with circulatory lubrication and oil cooling. High mix losses are shown also in these cases, on which the target maximum rotational speed of the interior gear is only possible conditionally. Under very high rotational speeds, the oil from the central part will be shot by the centrifugal force towards the housing, leaving the center of the transmission with a low oil level during the operation, generating frictional corrosion.
Operation of a planetary transmission with oil spray lubrication has also been tried. This is however not applicable for planetary transmissions, as the oil spray cannot reach the center of the transmission because of the strong ventilation effect caused by the high rotational speed of the interior gear, which means that the sun gear is not lubricated. Therefore the axial bearings will be insufficiently lubricated and frictional corrosion may even be generated.